1. Field of the Invention
The present invention relates to a semiconductor optical device, and to a method for fabricating a semiconductor optical device, and in particular to a semiconductor optical device which is adapted to form a low loss interconnection with an optical fibre.
2. Related Art
The development of semiconductor devices and optical fibres, for example for use in optical communications systems, has occurred very much in isolation from each other. Semiconductor devices, for example lasers, have been developed and optimised with their own particular requirements in mind, for example for lasers, low threshold, high bandwidth, narrow linewidth and high output power are all desirable. These requirements, in combination with the fundamental limitations imposed by the available semiconductor materials, in particular their refractive index, has meant that very small waveguide modes have been essential in order to optimise the functionality of these semiconductor optical devices. In contrast, the requirements for optical fibres have been the need for low loss, low material and waveguide dispersion, a range of mechanical properties (including being handleable), and the ability to be spliced. Although the progression from multi-mode to single mode fibres has seen a reduction in the waveguide mode size for optical fibres, the mode size for single mode fibres which in practice can be spliced, is very much larger than the waveguide mode sizes found in semiconductor optical devices. The spot size (1/e.sup.2 diameter) of standard telecommunications single mode optical fibre at a wavelength of 1.55 .mu.m is approximately 10 .mu.m, whereas the spot size at the output facet of a typical double hetrostructure semiconductor laser is 1.0 .mu.m by 1.5 .mu.m.
This large mismatch between the mode spot sizes of semiconductor optical devices and optical fibres means that coupling efficiencies between the two are very low, for example as low as 10% for a laser and a cleaved single mode fibre. In addition, tight alignment tolerances are required in order to maximise the coupling efficiency between a semiconductor laser and an optical fibre, which dramatically increases the cost of packaging semiconductor devices. One of the key economic requirements for the implementation of FTTH (fibre to the home) is the availability of low cost packaged semiconductor lasers. The largest proportion of the cost of the laser is incurred in packaging the device, and a major contribution to this cost is the need to use sub-micron active fibre alignment techniques to align the fibre to the semiconductor laser.
A technique for increasing the coupling efficiency between semiconductor devices and optical fibre is described in "Low Loss Coupling Between Semiconductor Laser And Single Mode Fibre Using Tapered Lensed Fibres" I W Marshall, British Telecom Technology Journal, Vol. 4, No. 2, April 1986. This paper describes how lenses can be formed on the end of single mode fibres by first drawing the fibre to a taper, by holding it under tension in the arc of a fusion splicer, and then forming a lens by melting the tip of the taper. While this technique does indeed increase the coupling efficiency between the fibre and the semiconductor laser, it significantly increases the cost of packaging these devices. In addition to the cost of actually forming the lens, higher alignment tolerances are required when such a lens is utilised, relative to those required with a cleaved fibre end. This arises because the sensitivity of the coupling loss to misalignment of the lensed fibre increases as the lens radius decreases, so that alignment tolerances must be traded for increased coupling efficiency.
An alternative approach to increasing the coupling efficiency between semiconductor optical devices and optical fibres, which also reduces the alignment tolerances required when packaging these devices, comprises the use of passive mode converters. Such devices are described in, for example, "Highly Efficient Fibre Waveguide Coupling Achieved By InGaAsP/InP Integrated Optical Mode Shape Adapters" presented at European Conference on Optical Communications 1993 by T Brenner and H Melchior and "Spot Size Converters Using InP/InAlAs Multiquantum Well Waveguides For Low Loss Single Mode Fibre Coupling" Electronic Letters 1992, Vol. 28, pp 1610-1617, N Yoshimoto et al. These devices are passive optical components which serve to optically connect single mode fibres to semiconductor devices by utilising tapered waveguide sections to transform the small semiconductor modes into the much larger mode of a single mode fibre. A very gradual change in the waveguide parameters of these passive mode converters is required to allow the optical mode to expand adiabatically, in order to avoid high losses. Although these passive mode converters do improve the coupling efficiency between semiconductor devices and optical fibres, and also increase the alignment tolerances, they comprise a further component which must be aligned and packaged between the semiconductor optical device and the optical fibre, and thus increase the complexity, size and cost of these packaged devices.
A laser having an integrated passive tapered waveguide which increases its output spot size is described in "Tapered Waveguide InGaAs/InGaAsP Multiple Quantum Well Lasers" T L Koch et al, IEEE Photonics Technology Letters, Vol. 2, No. 2, February 1990. This device employs a conventional multi-quantum well active layer, and a number of passive waveguide layers which are successively etched at different points along the length of the device in order to achieve a stepped, tapered waveguide. The active layer is evanescently coupled to the passive tapered waveguide, and a further passive "outrigger" waveguide spaced a considerable distance from the tapered waveguide, is also employed. The design allows little flexibility for the optimisation of the laser, for example the active layer is limited in thickness by the need to achieve a high degree of evanescent coupling to the passive tapered waveguide. This thickness limit on the active layer limits the high temperature performance of the laser. Furthermore the use of a stepped, tapered passive waveguide requires a large number of additional stages of photolithography, and thus considerably increases the complexity and cost of device fabrication.